Switchable Photoelectric Response in High-Temperature Leadless Molecular Ferroelectric [C 4 N 2 H 14 ][BiI 5 ].

Lead-free molecular ferroelectrics have garnered considerable attention for their promising potential, but such species with narrow band gap and sensitive photoelectric response are yet inadequate. Herein, we demonstrated the bulk ferroelectric photovoltaic effect in a novel lead-free molecular ferroelectric [C 4 N 2 H 14 ][BiI 5 ] with a Curie temperature ( T c ) of 366 K and a narrow band gap ( E g ) of 1.92 eV. The transformation of the crystal structure from the polar space group P 2 1 to the nonpolar space group P 2 1 / m was elucidated using single-crystal X-ray diffraction. Room-temperature (RT) hysteresis loop reveals the intrinsic ferroelectricity of [C 4 N 2 H 14 ][BiI 5 ] with a relative small coercive field ( E c ∼ 0.27 kV/cm), saturation polarization ( P s ∼ 1.87 μC/cm 2 ), and remanent polarization ( P r ∼ 1.61 μC/cm 2 ). [C 4 N 2 H 14 ][BiI 5 ]-based solar device exhibits significant PV effects with a steady-state photocurrent ( J sc ) of 3.54 μA/cm 2 and a photovoltage ( V oc ) of 0.34 V under AM 1.5 G illumination, which can be significantly improved by adjusting the ferroelectric polarization, reaching a maximum J sc of 140 μA/cm 2 and V oc of 0.51 V. This work offers a promising avenue for lead-free molecular ferroelectric materials in the field of optoelectronic devices.